If they are the back 2 plugs on both sides of the engine the problem is the secondary float level on the back two barrels of the carb., if it's the front 2 plugs on each side it's the primary float level. The needle and seat valve is likely stuck open allowing fuel to drip into the manifold, flooding the 4 cylinders. My Holley did the same thing last year, which is fairly common on Holleys.

Take off the air cleaner and look into the carb., if you see fuel dripping in the back two barrels, or the front two barrels you have identified the problem. Here are some video clips I posted showing the problem.